Shoe presses generally comprise a press shoe and a counter roll which form an extended nip therebetween through which a running fibrous web is carried for treating the web, such as for dewatering the web in the press section of a paper machine. Shoe presses generally also comprise pressure-actuatable piston-and-cylinder units, also referred to as loading cylinders, which are distributed along the press shoe in one or more rows in the longitudinal direction of the press shoe and adapted to press the press shoe against the counter roll. For example, EP 345 501 B2, DE-195 15 832 C1, and DE-44 09 316 C1 all show shoe presses of the type described above.
Some shoe presses also include compartments arranged in the press shoe surface facing the counter roll, the compartments in operation being supplied with fluid under hydrostatic pressure for lubricating the belt, as shown for example in EP-345 501 B2.
Moreover shoe presses may comprise a pocket or pressure chamber between each loading cylinder and the press shoe, which may be open towards the press shoe and in operation be supplied with fluid under hydrostatic pressure. The pressure chamber acts to transmit the pressure of the loading cylinder to the press shoe and to form a hydraulic fluid pad, as disclosed for example in DE-195 15 832 C1.
These loading cylinders, compartments, and pockets require access to hydraulic fluid. Various designs have been developed for delivering hydraulic fluid to the working chambers of loading cylinders, to hydrostatic compartments in a press shoe surface for belt lubrication, and to hydrostatic pockets between loading cylinders and a press shoe. For example, EP-345 501 B2 discloses a shoe press in which the compartments in the press shoe surface facing the counter roll are pressurized by hydraulic fluid through a main duct which is common to all the compartments and is bored through the frame system of the shoe press in the longitudinal direction (i.e., cross-machine direction) of the shoe press. A plurality of individual ducts bored in the frame connect the main duct to each compartment, each of the individual ducts serves as a throttle. The throttle serves the purpose of making the pressure condition of each compartment essentially independent of the pressure conditions of the other compartments. EP-345 501 B2 also discloses that the loading cylinders are pressurized by hydraulic fluid via additional ducts bored in the frame system of the shoe press.
Similarly, DE-195 15 832 C1 also discloses a shoe press in which ducts are bored in the frame system of the shoe press for supplying hydraulic fluid to the loading cylinders by hydraulic fluid as well as for pressurizing the pressure chambers between the press shoe and the loading cylinders, a cylinder member of each loading cylinder including an opening or throttle for passing fluid from the working chamber of the loading cylinder into the pressure chamber.
The frame system of a shoe press is typically formed of relatively massive members, for example steel beams. Accordingly, boring ducts through the frame system for supplying hydraulic fluid to the various chambers and compartments is a complicated and expensive procedure. Additionally, the fixed geometry of the ducts does not readily allow for varying the flow characteristics, for example where it is desired to change the relative proportions of flow supplied to various ones of several chambers or compartments.
In addition, shoe presses are subjected to thermal action, for instance owing to the friction between the circulated flexible belt and the press shoe. Different parts of the shoe press, for instance, the press shoe and the frame system, expand at different speeds and to different degrees when subjected to heat, which causes stress and deformation problems as well as jamming of the loading cylinder pistons in the cylinder parts thereof. The problem is exacerbated if the parts of the shoe press are made of different materials, e.g., steel for the frame system and aluminum (alloy) for the press shoe.